1. Field of the Invention
The present invention relates to a battery electrode for use in a secondary battery and a producing method thereof Particularly, the present invention relates to an electrode for a nickel-hydrogen battery and a producing method thereof.
2. Description of the Background Art
A secondary battery that can be charged for reuse is used as a power source for an electronic apparatus, particularly for a portable apparatus. Typical examples of such secondary batteries include alkaline secondary batteries such as nickel-cadmium battery, nickel-hydrogen battery and the like. Among the alkaline secondary batteries, particularly, a high-capacity nickel-hydrogen battery has attracted attention.
The nickel-hydrogen battery is used in a wide range of applications, as it is highly reliable and has a long life, and is less expensive and can be reduced in size than a lithium-ion battery. In particular, hybrid vehicles have recently come into practical use by vehicle manufacturers from the standpoints of energy saving and environmental protection, and have widely attracted attention in foreign countries as well. Currently, the nickel-hydrogen battery is predominantly used as the power source for the hybrid vehicle.
An alkaline battery, often used as a power source for various kinds of devices ranging from portable apparatuses to industrial large-scale facilities, uses a nickel electrode as a positive electrode in most cases. Such a nickel electrode has a structure wherein a collector having an electricity-collecting function carries an active material (positive electrode active material) for causing cell reaction. For the collector, a pocket-type collector was conventionally employed. With development of a sintered nickel plate (nickel sintered body) obtained by sintering nickel powder, however, the nickel sintered body came to be used as the collector, which promoted the practical use of the alkaline secondary batteries.
Subsequently, it was tried to lower the cost and increase the capacity of the nickel electrode. As a way of lowering the cost, it was proposed to use a collector of a two-dimensional structure, such as punching metal, instead of the nickel sintered body having a three-dimensional structure. Specifically, a nickel electrode is fabricated by filling a pasty active material in pores of the punching metal, although this technique has not come into practical use yet.
The increase of the capacity of the nickel electrode has become possible by using, instead of the nickel sintered body, foamed nickel also having a three-dimensional structure as the collector. Generally, the foamed nickel is produced in the following manner. A sheet-shaped foamed article of urethane resin is coated with nickel plating, and is annealed in a reducing atmosphere after incineration of the urethane resin. This can increase the strength of the nickel framework. The foamed nickel is then filled with an active material paste and pressed to obtain a nickel electrode. The foamed nickel has a very large porosity of 92% to 96%, while the above-described nickel sintered body has a porosity of about 80%. This means that, compared to the nickel sintered body, the foamed nickel can increase the amount of the active material that can be filled in per unit volume, which leads to an increase of the capacity.
At the initial stage of development, the foamed nickel was susceptible to damages. For example, at the time of winding a sheet-shaped nickel electrode and housing it in a cylindrical battery case, the foamed nickel would likely suffer cracking. At present, however, this problem has been solved, and the cylindrical as well as prismatic nickel-hydrogen batteries using the foamed nickel as the collector have come into practical use, not only for portable apparatuses, but also for the hybrid vehicles requiring high power output and high reliability. As described above, although the nickel sintered body is still used as the collector of the nickel electrode for the nickel-hydrogen battery in some apparatuses, the nickel electrode having the foamed nickel as the collector that is filled with an active material has become dominant.
Currently, the foamed nickel has reached a level having characteristics suitable for high power application as well as for high capacity application as the collector. The remaining problems are that the manufacturing process is a little complicated, and that it is desirable to reduce the amount of nickel being plated, if possible. In other words, there is a demand for reduction of cost by simplification of the process and reduction in amount of the base material.
Under these circumstances, in recent years, there is proposed an electrode that is produced by employing a porous electrode substrate (collector) formed by plating woven or non-woven fabric with metal (typically, nickel), and by filling an active material in the pores formed in the porous electrode substrate. Here, the active material thus filled is settled in the pores of the produced electrode to work as the active material. The porous electrode substrate is advantageous in that the manufacturing process is easier and the amount of the metal being used can further be reduced, compared to the foamed nickel.
As such an electrode formed by filling an active material in a porous electrode substrate, for example, Japanese Patent Laying-Open No. 55-030180 (Patent Document 1) discloses an electrode wherein an electrode base (porous electrode substrate) is prepared by metal-plating a porous body made of natural or synthetic fiber, and the electrode substrate is filled with an active material to produce the electrode. Further, Japanese Patent Laying-Open No. 61-208756 (Patent Document 2) discloses an electrode for an alkaline secondary battery, wherein a fiber mat having a metal-plated surface is prepared as an electrode base (porous electrode substrate), and the electrode substrate is filled with an active material and then pressed to obtain the electrode.